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INTRODUCTION CableLabs®: a number of cable operators create Cable the Television Laboratories DOCSIS (Data Over Cable Service Interface Specification): a standard for data transmissions on CATV/HFC networks

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INTRODUCTION CATV :It started to distribute broadcast video signals to locations with poor or no reception in the late 1940s,it was called community antenna TV (CATV) HFC: A system with fiber for the long-haul runs and coaxial cable to the houses is called an HFC(Hybrid fiber-coaxial ) system

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Physical layer: includes the modulation schemes used on the coax network For DOCSIS, these are 64-quadrature amplitude modulation (QAM) and 256-QAM on the forward quadrature phase shift keying (QPSK) and 16QAM on the reverse

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DOCSIS PROTOCOL STACK MPEG-2 Transmission Convergence layer: DOCSIS CM data is encapsulated in 188-byte MPEG-2 frames This allows the data to be multiplexed with other MPEG streams on the same carrier on the forward path

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DOCSIS PROTOCOL STACK Media access control (MAC) layer: Controls CM access to the return path there maybe many CMs trying to transmit simultaneously on the return path MAC protocol provides an orderly method for the CMTS to tell a CM when it gets to transmit and for how long

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DOCSIS PROTOCOL STACK Data link encryption layer: there has to be a method to protect user data from malicious users

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DATA TRANSMISSION ON MULTIPLE UPSTREAM CHANNELS In DOCSIS, an upstream channel is shared by many stations An upstream channel is a TDMA type shared channel

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DATA TRANSMISSION ON MULTIPLE UPSTREAM CHANNELS Channel will divide into fixed length time slots called minislots A series of minislots to be used for reservation requests, called a request cluster Before transmission, the station must send a request in a request cluster

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REQUEST CLUSTER ALLOCATION SCHEME Binary Exponential Back-off scheme The request cluster size is set to a power of 2,Say 2^x X is a positive integer called the back-off window Each station generates one of the values from 0 to (2^x-1) If two or more stations generate a same number, a collision occurs

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Too large: if only one collision occurs when the request cluster is large, say 512, then it allocates 1024 minislots for the next round Useless request cluster would increase Stations which want transmission data will wait the large request cluster goes past

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REQUEST CLUSTER ALLOCATION SCHEME Too small: When collisions have been resovled, the request cluster is set to the minimum value When under a heavy-load condition, collisions occur many times, there will make many stations fail reservation and try again

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REQUEST CLUSTER ALLOCATION SCHEME Proposed Scheme R:the number of request minislots safely received in the request cluster C:the number of collided minislots in the request cluster